cifs: Unset CIFS_MOUNT_POSIX_PATHS flag when following dfs mounts

In a dfs setup where the client transitions from a server which supports
posix paths to a server which doesn't support posix paths, the flag
CIFS_MOUNT_POSIX_PATHS is not reset. This leads to the wrong directory
separator being used causing smb commands to fail.

Consider the following case where a dfs share on a samba server points
to a share on windows smb server.
 # mount -t cifs -o .. //vm140-31/dfsroot/testwin/
 # ls -l /mnt; touch /mnt/a
 total 0
 touch: cannot touch ‘/mnt/a’: No such file or directory

Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Acked-by: Jeff Layton <jlayton@samba.org>
Signed-off-by: Steve French <steve.french@primarydata.com>